Various programmable logic architectures are known, including, for example, Programmable Logic Devices ("PLD"), Programmable Logic Arrays ("PLA") and Programmable Array Logic ("PAL.TM."). Although there are many differences between the various architectures, each of the PLD, PLA and PAL architectures typically include a set of input conductors directly coupled as inputs to an array of logical AND gates ("Product Term Array"), the outputs of which, in turn, act as inputs to another portion of the logic device.
FIG. 1 shows a conventional Complex Programmable Logic Device ("CPLD") 100 which includes a Programmable Interconnect Matrix ("PIM") 110 and eight logic blocks 120. Other CPLDs may include different numbers of logic blocks. As shown, CPLD 100 has a total of 262 inputs, each of which is connected to the PIM 110. The PIM 110 is capable of providing each logic block 120 with its own set of input terms by independently selecting as many as 36 of the possible 262 input signals as input terms for each logic block 120. The compliments for each of the 36 signals output by the PIM 110 are also provided to each logic block 120. Thus, each logic block receives as many as 72 input terms from the PIM 110.
The PIM 110 includes a number of programmable elements (not shown) for controlling an array of multiplexers (not shown) to reduce the total number of programmable elements required. The programmable elements may be volatile memory elements such as static random access memory ("SRAM"), non-volatile memory elements such as flash electrically easrable programmable read only memory ("Flash EEPROM"), or fuses. Alternatively, the programmable elements of the PIM 110 may be implemented to control a matrix of cross point switches; however, this increases the total number of programmable elements required to make the same number of connections. Whether the connections between input conductors and output conductors are provided by cross point switch matrix or by an array of multiplexers, each output conductor can be connected to at least one input conductor. Thus, both forms of connections perform a multiplexing function in the sense of both forms of connection provide for the selection of one of the input conductors from the set of many input conductors.
The 262 inputs to the PIM 110 include 128 feedback signals, 128 input signals and 6 dedicated input signals which include 4 clock signals. Sixteen feedback signals and as many as 16 input signals are provided by each logic block 120. Each logic block 120 is programmed to perform selected logic functions using sub-combinations of the 72 input terms provided by the PIM 110. Each logic block 120 has 16 input/output ("I/O") pins 125, which may be used as either inputs to the PIM 110 or outputs of the CPLD 100.
Conceptually, the CPLD 100 may be regarded as a PIM coupled in series with 8 PLDs coupled in parallel, where each individual logic block 120 corresponds to a single PLD. Intermediate stages in the outputs of each of the 8 PLDs are fed back as inputs to the PIM. Depending on the particular set of input signals routed to the outputs of the PIM and the programmed logic functions for each logic block 120, the 8 PLDs may, in fact, act as two or more PLDs coupled in series with each other. CPLD 100 thus provide a highly versatile logic device.
FIG. 2 shown portions of CPLD 100 in greater detail. Specifically, a logic block 120 is shown as including a product term array 210, a product term allocator 215, macrocells 220 and I/O cells 225. The product term array 210 may be a fully programmable AND array and the product term allocator 215 is configured to allocate product terms from the product term array 210 to 16 macrocells 220. The product term allocator 215 "steers" product terms to macrocells 220 as needed. For example, if one macrocell requires ten products terms while another requires only three product terms, the product term allocator 215 steers ten product terms to one macrocell and three product terms to the other macrocell. Depending on the configuration, up to 16 product terms can be steered to any one macrocell.
The output for each of the 16 macrocells 220 are fed back to the PIM 110 as input signals. The specific architecture of the macrocells 220 may be any appropriate architecture and each macrocell 220 may include registers and/or buffers. The 16 outputs of the macrocells 220 are also fed to 16 I/O cells 225. Each of the 16 I/O cells 225 can be programmed to provide output signals to an I/O pin 125 or input signals to the PIM 110.
CPLD 100, as shown FIGS. 1 and 2, provides a highly versatile programmable logic device. However, as the need for implementing more and more logic functions within digital and/or computer systems has increased, CPLDs of even higher densities have been developed. For example, as shown in FIG. 3, CPLD 300 consists of logic blocks connected by two levels of interconnect. Each group of four logic blocks 310 is given its own routing resource, called a Block Interconnect 320. Together, the four logic blocks 310 and their block interconnect 320 are called a segment 330. The second level of interconnect, segment interconnect 340, ties all of the segments 330 together. Segment interconnect 340 also provides a routing path for a number of global clock signals 350.
Each logic block 310 consists of macrocells, logic arrays, logic allocators, I/O cells and any necessary control signal generators. Thus, each logic block 310 resembles an independent PLD device. In addition to the block interconnect 320, I/O cells associated with each logic block 310 may have signal paths which feed directly back to that logic block (local feedback paths) which may be used to route signals from the I/O cell back to the logic block. When the signals from an I/O cell are used in another logic block, an interconnect feeder may assign a block interconnect line to that signal. The interconnect feeder may act as an input switch matrix.
The block interconnect 320 and segment interconnect 340 provide connections between any two signals in CPLD 300. Block interconnect lines and local feedback lines are assigned as required.
Although CPLD 300 provides a high degree of flexibility for implementing logic functions, it also suffers from certain drawbacks. For example, segment interconnect 340 and block interconnect 320 are each multiplexer-based interconnect structures. As such, segment interconnect 340 and block interconnects 320 require significant die area, thus leading to increased fabrication costs for CPLD 300. Segment interconnect 340 is required to be multiplexer-based because of the need to provide interconnection between every logic block 310 of CPLD 300. Although such a scheme may provide maximum connectability, it tends to ignore "real-world" designs which typically implement related logic functions in related logic blocks of a segment, thus avoiding the need to transmit numerous signals across an interconnect between segments.